The invention relates to method for deriving and respectively for calibrating a frequency of a pulse signal. The method is used in electronic devices, preferably in electricity meters, supply meters, ripple-control receivers, or combinations of these devices.
A method of the type descried hereinabove is known from U.S. Pat. No. 5,644,271 in which the compensation for frequency drift of a digital pulse signal caused by temperature is described, which pulse signal is generated by an oscillator and the frequency of which is temperature-dependant. The temperature is measured using a sensor, for example, a thermistor, the analog output signal of which in converted into a digital signal using an analog to digital converter. The digital value of the latter serves as an address to a memory in which corrective values for the frequency as a function of temperature are stored in table form. The data output of the memory is supplied via an accumulator to a delay device which, for example, is composed of a counter, the pulse input of which is supplied by a reference frequency pulse signal which is generated by a reference quartz oscillator. The counter counts the pulses of the latter. The time delay of the delay device can be adjusted digitally, in that a corrective value is loaded in parallel into the counter which then, starting with the corrective value, counts backwards to zero and on reaching the zero value generates an output pulse. The output signal of the delay device is consequently composed of temperature-dependent delayed pulses, which form the temperature-compensated pulse signal.
U.S. Pat. No. 4,902,964 is also known, in which an electricity meter with a demand register is described. Said demand register registers consumption data accumulated and stored over programmed time periods using a microprocessor. In this case knowledge of the accurate real time is of essential importance. For this purpose, a pulse signal needed for the microprocessor is derived from a 50 Hz or 60 Hz line frequency, wherein periods of a line signal are determined with the aid of a reference frequency generated using a quartz oscillator. A counter determines the line periods in that it counts the pulses of the reference frequency present between two consecutive zero crossings of the line frequency. A frequency comparator subsequently establishes whether the line period determined is a 50 Hz or a 60 Hz period, taking into account the permitted frequency tolerances, whereupon the counted value of an accompanying 50 Hz or 60 Hz counter is incremented. As soon as the counted value of one of these counters has reached the value of five, which corresponds to five zero crossings of the frequency concerned, the operating parameters of the microprocessor and of the program pulse signal are automatically established according to the line period determined.
The object of the invention is to provide a method of the type described in the introduction that makes it possible to automatically compensate for temperature and/or non-temperature-dependent undesired changes in the frequency of a pulse signal in a simple and inexpensive manner and with little complexity of circuitry, using available components where possible and without a second oscillator also being necessary. Such components already present in electricity meters, supply meters, ripple control receivers or combinations of these devices are, for example, dividers having a division factor of two.
The object described is solved in accordance with the invention by generating the frequency of the pulse signal by using the line frequency when measured periods of the line frequency pulse signal are within predetermined tolerance limits, and otherwise deriving the frequency of the pulse signal from an oscillator frequency. Advantageous embodiments of the invention are evident from the following detailed description and accompanying drawings.
An embodiment of the invention is shown in the drawings, and will be described in more detail hereinafter.
There is shown, in: